1. Field of the Invention
The present invention relates to a focus detecting apparatus which can be applied to an auto-level, a theodolite or a transit instrument etc., having a telephotographic system.
2. Description of the Related Art
A surveying instrument, such as an auto-level, a theodolite or a transit instrument etc., is provided with a collimating telescope, a level, and scales for measuring a rotative angle and/or an elevational angle. The collimating telescope is first horizontally set and is then adjusted horizontally and vertically, so that an operator may observe a collimation point or object.
A typical surveying instrument having a collimating telescope is provided, in order from the object side, with an objective lens, a focusing lens and an eyepiece. The position of the focusing lens is adjusted according to the distance from the object, so that an image of the object may be formed on a focusing plate (reticle). The operator may thus observe, via the eyepiece the image superimposed on the reticle.
Some types of surveying instruments are provided with a so-called passive type of automatic focusing apparatus. As is well known, the focusing of the automatic focusing apparatus is performed in the following manner. First, an optical path of the optical system of the collimating telescope is split into two. Then one of the split optical paths is further split into two at a focus detecting sensor provided along the split optical path. An object image is formed on a pair of line sensors positioned at a point equivalent to that of the focusing plate (reticle). Defocus is detected according to the phase difference of the object images on the pair of line sensors, and focusing is performed.
Generally, since a surveying instrument is used in conjunction with a staff positioned at the center of the visual field, it is desirable that the object image used to detect the defocus has a center portion at an intersection point of the reticle. However, when the described focus detecting sensor is provided to the surveying instrument, the center of the focus detecting sensors may deviate due to errors in mounting or in manufacturing. The deviation of the defocus detecting portion from the center may result in the formation of an image portion which is not properly positioned with respect to the reticle. The focus detecting sensor may be moved slightly in order to facilitate centering, but such a movement is difficult to enable both in structure and in operation.
The surveying instrument normally utilizes a staff as the aiming object. Accordingly, the size (width) of the staff image on an object forming (focusing) plate decreases as the distance increases, as shown in FIGS. 8 and 9. Consequently, the focal point may be detected with respect to an image near the staff image. The width of the focus detecting portion may narrow in the case of a long distance, but when the width of the focus detecting portion is narrowed in the aforementioned case of deviation from the center, it is impossible to detect the focusing state of a thin object image positioned at the center of the visual field.